1. Field of the Invention
The present invention relates generally to relief devices for tanks containing high-pressure gasses. More specifically, the present invention relates to relief devices for truck-borne compressed gas cylinders.
2. Description of the Related Art
Various types of compressed gases are commonly transported in long, narrow cylinders, or "tubes," mounted directly on a tractor trailer chassis or in a "module," a box frame containing the cylinders that is loaded onto a flat bed trailer. These truck-borne compressed gas cylinders are typically fitted at each end with cluster-type relief devices that are designed to relieve pressure from compressed gas cylinders in the event of cylinder over-pressurization or fire. The present invention deals with an improved form of a cluster-type relief device.
FIGS. 1 and 2 illustrate a prior art cluster-type relief device. FIG. 1 is an exploded view of a prior art cluster-type relief device's interaction with its associated compressed gas cylinder, illustrating the cylinder 41, an "O" ring 47, a backup ring 48, a "bullplug" 42, the prior art cluster-type relief device 40, and a valve 49. As shown in FIG. 1, the relief device 40 does not attach directly to the cylinder 41. Rather, the relief device 40 screws into bullplug 42, which in turn screws into the cylinder 41. (The "O" ring 47 and backup ring 48 help create a seal between the bullplug 42 and the cylinder 41.) Thus, the bullplug 42 has two sets of threads: a set of male straight threads 43 that engage a reciprocal female set 44 in the cylinder; and a set of female pipe threads 45 that engage a reciprocal set of male pipe threads 46 on the relief device 40. Each set of threads 43, 45 on the bullplug 42 represents a potential leak or failure point. Moreover, having the relief device 40 screw into the bullplug 42, which in turn screws into the cylinder 41, means that the relief device protrudes farther from the cylinder than it would if the relief device screwed directly into the cylinder. This greater protrusion presents a greater opportunity for a shear of the relief device from the cylinder in the event of an accident, resulting in uncontrolled compressed gas leakage.
FIG. 2 is a close-up exploded view of a prior art cluster-type relief device. As shown in FIG. 2, the prior art relief device comprises a main body 55 and sets of washers 50 (typically made of silver, copper, or other suitable metal), rupture discs 51, and cluster nuts 52. The operation of the cluster nuts 52 and their associated rupture discs 51 is described below--prior art cluster-type relief devices and the present invention operate to relieve pressure in essentially the same manner. These relief devices typically have three, five, or six sets of cluster nuts 52, washers 50, and discs 51. FIG. 2 illustrates a device with six such sets--one for each face of its hex head.
FIG. 2 illustrates how the cluster nuts 52 protrude beyond the diameter of the main body 55. This protrusion presents yet another opportunity for shear in the event of an accident, which in this case is the shear of the cluster nut head away from its threaded portion. In the event of a cluster nut head shear, the rupture disc and the threaded portion of the cluster nut will remain intact and in place within the relief device body. However, a cluster nut head shear greatly increases the likelihood that the relief device will relieve prematurely or accidentally outside of its design parameters.
Therefore, a need exists for a cluster-type relief device that attaches directly to its associated compressed gas tank, rather than via a bullplug, thereby lessening the likelihood of 1) leakage past a threaded connection and 2) relief device shear--and the associated uncontrolled leakage--in the event of an accident. Furthermore, a need exists for a cluster-type relief device with cluster nuts that do not protrude beyond the diameter of the device body, lessening the likelihood of cluster nut shear and the associated increased likelihood of premature relief.
Accordingly, the present invention provides a cluster-type relief device for compressed gas cylinders that attaches directly to its cylinder rather than threading into a bullplug. It is a further object of this invention to provide a cluster-type relief device for compressed gas cylinders that protects its cluster nuts from shear in the event of an accident. Thus, the present invention features a larger diameter threaded connection that may be threaded directly into the cylinder in place of a bullplug. Moreover, the present invention's cluster nut heads are recessed into the relief device body, thereby greatly reducing or eliminating the chance of accidental cluster nut head shear.